Vehicle coasting is a vehicle state or mode of operation in which a motor vehicle operates with a clutch used to connect an engine of the motor vehicle to a driveline of the motor vehicle in a disengaged state so that the motor vehicle moves without being decelerated by engine drag. During such coasting it is usual for the engine to be operated at idle speed in order to save fuel and reduce emission although, in some cases, the engine is automatically stopped during coasting.
The transition from a driving mode such as vehicle acceleration to coasting or vice versa requires several steps to be executed and, if the motor vehicle is equipped with an electronically controlled clutch (E-clutch), the transition can be automated in a process known as ‘autonomous coasting’ in which the driver does not actively produce the conditions required for coasting but rather the actions of the driver are used to automatically trigger the entry and exit from a state of coasting.
To meet customer drivability expectations, driveline surge or shuffle caused by torsional oscillations in the driveline during such an automatic transition need to be avoided. This requires a control strategy to be used designed to reduce such shuffle or torsional oscillations, however, the design of such a control strategy is not a trivial matter because it must take into account both driveline backlash and driveline compliance.
Driveline backlash consists of free travel between gears and other parts throughout the driveline such that changing the direction of rotational movement will result in lost motion referred to as the ‘lash zone’. When in the lash zone the driveline is said to be in a lash state and no torque can be transmitted by the driveline as the driveline travels through a lash zone. However as soon as the driveline lash zone has been crossed a large torque is immediately transmitted by the driveline and this abrupt change in transmitted torque results in a system response that will result in shuffle unless it is controlled by the control strategy.
Driveline compliance increases the control challenge because the application of torque to the various shafts of the driveline cause them to get twisted or wound up and this wind-up has to be released in a controlled manner in order to avoid uncontrolled travel through the lash zone when transitioning from positive wind-up to negative wind-up or vice-versa.
It is a problem that, in the case of a powertrain having a manual transmission and an E-clutch an automated transition from one driveline mode such as acceleration to coasting or vice versa will, due to driveline backlash and driveline wind-up, result in an uncontrolled transition causing jerks and longitudinal vehicle shuffle if a control strategy taking no account of wind-up and compliance is used to effect the transition.